The ability to vary the power between an input and output shaft is vital to industries and economies throughout the world. Industries dependent on variable power transmission range from energy exploration and power generation to transportation and construction. Consequently, the applications range from stationary to mobile equipment, but the desired result remains the same, that is, to achieve the desired output of torque or speed in the most efficient manner possible.
In order to achieve these desired power transmission results a number of systems have developed over the years to vary the desired rotational speed output with respect to the prime input member in the most efficient manner possible. Most, if not all, such current systems may be classified as either stepped, conventional power transmission systems or step-less, continuously variable power transmission systems. Each of the presently available systems, whether conventional or continuously variable, have distinct advantages and corresponding disadvantages associated therewith.
First, conventional power transmission systems employ the use of multiple gear sets and clutching devices. Such systems, typically, receive input from a single source, and the speed ratio changes are accomplished in discrete steps by engaging different gears in the power transmission pathway until the output is in the vicinity of that which is desired. The output speed variation between two of the “geared” speed ratios is obtained by varying the input speed supplied by the prime mover. Consequently, the prime mover cannot always operate at its most efficient speed, resulting in a less than ideal power transmission system.
To the contrary, continuously variable transmission systems provide continuously variable speed ratio change between the minimum and maximum available speed ratios. With this type of power transmission system, the prime mover may be operated at its optimum speed for peak performance or efficiency. Presently available continuously variable transmission systems include belt systems, toroidal systems, and hydrostatic systems. These present continuously variable transmission systems provide a significant advantage over conventional systems; however, these systems are not without their own drawbacks.
Belt driven continuously variable transmissions consist essentially of a drive pulley, a belt, a driven pulley, and a control system. The drive pulley is driven by the prime mover and consists of two cones facing each other. The driven pulley transfers power to the output, and it also consists of two cones facing each other. The belt rides in the groove between the two cones of each pulley. When the two cones of the pulley are far apart (when the diameter increases), the belt rides lower in the groove, and the radius of the belt loop going around the pulley gets smaller. When the cones are close together (when the diameter decreases), the belt rides higher in the groove, and the radius of the belt loop going around the pulley gets larger. Such a continuously variable transmission system may use hydraulic pressure, centrifugal force, or spring tension to create the force necessary to adjust the pulley halves. This type of system works well for its intended purpose and provides many advantages including its efficiency and simplicity; however, several drawbacks of the belt driven continuously variable transmission exists as well. First, this type of system is typically limited to small, relatively low horsepower applications because of its reliance on the belt for full power transmission. In such a system, the belt can stretch (resulting in slippage and efficiency loss) or break resulting in complete power failure. Additionally, the system is limited by its size. The typical belt system is large in size and weight, limiting its useful applications to light stationary or light mobile equipment.
Toroidal continuously variable power transmissions works similarly to the belt system, but it replaces the belt and pulleys with discs and power rollers. The input disc is driven by the prime mover, and the output disc transfers power to the output. Rollers are located between the discs acting like the belt, in a belt system, transmitting power from the input disc to the output disc. In operation, the rollers can rotate along two separate axes. Each roller may spin around the horizontal axis and tilt in or out around the vertical axis, which allows the roller to contact the discs in different areas. When the rollers are in contact with the input disc near the center, they must contact the output disc near the rim, resulting in a reduction in speed and an increase in torque. When the rollers contact the input disc near the rim, they must contact the output disc near the center, resulting in an increase in speed and a decrease in torque. Therefore, any tilt of the rollers incrementally changes the gear ratio, providing for an infinite variation in speed ratios between the corresponding system's minimum and maximum ratio. This type of system, similarly to the belt system, suffers from drawbacks associated with its limited size and scope. Toroidal continuously variable power transmissions are unable to handle large torque loads, and are quite heavy, limiting it to light stationary and mobile equipment as well.
Finally, hydrostatic continuously variable transmission systems use variable displacement pumps to vary the fluid flow into hydrostatic motors. In this system, the rotational motion of the prime mover operates a hydrostatic pump on the input side. The pump converts the rotational motion into fluid flow; then, with a hydrostatic motor located on the output side, the fluid flow is converted back into rotational motion. However, hydrostatic drives also have several drawbacks. The hydrostatic power transmission systems are noisy and operate at very low efficiency. Therefore, they are generally used only for low speed applications such as agricultural machinery and construction equipment. Additionally, hydrostatic power transmission systems are prone to contamination, which can result in efficiency loss or catastrophic system failure.
More recent developments in step-less, continuously variable power transmission systems involve the use of electromechanical transmission systems. Many such systems operate on a power-split concept similar to hydrostatic drives. Furthermore, the typical electromechanical power transmission system integrates either single or compound planetary gear trains to achieve a continuously variable transmission of power. However, a number of inherent deficiencies exist in this type of mechanical gear train that are well known in the art. For instance, the efficiency and performance of this type of system is detrimentally impacted by the sliding frictional forces generated during its operation. In order to transfer torque, planetary gear systems depend on the sliding engagement of individual gear teeth. It is well known that this sliding produces high frictional forces between the teeth, which can lead to total destruction of the system if not continuously and properly lubricated. Furthermore, proper transfer of torque in these planetary gear systems is totally reliant on the strength of each individual gear tooth. As the input member of the system rotates at a given torque, the force from each single tooth of the input is transferred, one at a time, to each single tooth of the mating gear. As a result, each individual tooth must be designed to transfer the entire force of the system including any impact loads that may be introduced at any particular time. Additionally, any tooth breakage can lead to catastrophic failure of the entire system. Finally, traditional means of manufacturing housing and components of current planetary gear systems are not only expensive and time consuming to set up and modify, but they are also expensive and time consuming to manufacture and produce. The housing for such a system consists of two or more cast parts assembled together; therefore, in order to either originally produce housings or modify existing designs, either new molds must be manufactured or modifications must be made to existing molds. Likewise, expensive tooling and highly skilled personnel are required for both the gears themselves and other major components of a planetary gear system.
In view of the limitations of products currently known in the art, a tremendous need exists for a continuously variable transmission system that is compact, efficient, durable, reliable, cost-effective, and able to handle high power applications.